This invention relates to semiconductor device power modules and more specifically relates to a novel structure for such devices which reduces their manufacturing cost and increases their reliability.
Semiconductor modules are well known, in which a plurality of power semiconductor die are fixed to a substrate support of an insulation metal substrate (IMS) or the like to interconnect the devices. Dual bonded copper (DBC) substrates can be used in place of IMS. The substrate support is carried in a main support shell which also supports a printed circuit board (PCB) which carries control circuits for controlling the power die. Power terminals extend from the IMS for connection to a load, such as a motor and a PCB carries a terminal connector for connection to an external source of control signals. Such devices, as shown in aforementioned application Ser. No. 09/197,078 are usually arranged so that the IMS is secured within a small opening in the shell (so that the area of the expensive IMS can be minimized) and the bottom surface of the IMS can be pressed into contact with the top flat surface of a heat sink.
The PCB is generally supported in a plane above the plane of the IMS and is laterally removed from the IMS area. The bottom of the PCB is thus spaced above the top surface of the support shell so that components can be mounted on the bottom surface of the PCB as well as on its top surface.
As a result of this structure, wire bonds to the control electrodes of the power die on the IMS, for example, gate electrodes and temperature and current sense and Kelvin electrodes of MOSFETs and IGBTs, must extend from the lower plane of the top surfaces of the power die to the upper plane of the upper surface of the PCB, creating long wire bonds which are difficult to manage.
Further, in the prior art structure, a substrate, usually IMS, containing the interconnected power semiconductor die, shunts, temperature and current sensors is first attached to an insulation base shell. A PCB is next attached to the base shell and wire bonds are made between the silicon die and substrate to the PCB. A cap is next placed over the IMS and encapsulant, for example, a silicone is introduced into the interior of the cap and atop the IMS through openings in the cap, and the silicone is cured. It would be advantageous to reduce the part count for the module.
In accordance with a first feature of the present invention, the support insulation shell structure is modified to support the IMS in a higher plane above the bottom of the shell and closer to the plane of the PCB. The main heat sink to receive the module is also modified to have a raised flat topped mesa to engage the raised bottom surface of the IMS. Thus, the difference in height between the IMS (or other similar substrate) and the PCB is reduced and they are in closely adjacent parallel planes. By xe2x80x9cclosely adjacentxe2x80x9d is meant less than about twice the thickness of the IMS.
This novel structure produces a number of advantages. First, the reduction in the height differential of the tops of the die on the IMS and the top of the PCB improves wire bondability and the quality of the wire bonds, thus improving production yield.
Second, the length of wire bonds is reduced, and mechanical stress on the wirebonds during device operation is reduced.
Third, the volume of the cavity that needs to be filled by encapulant above the IMS is reduced, reducing the volume of potting material used.
In accordance with a second feature of the present invention, the substrate carrying the power die and current and temperature sensors, shunts and the like is attached directly to and supports the PCB and the conventional insulation base shell is eliminated. The PCB has suitable openings to expose the top of the IMS substrate, leaving accessible wire bonding locations for bonding between the silicon substrate and the PCB. A cap is next mounted on top of the assembly and is secured by adhesive or by a screw structure. The cap is pressed toward the surface of the heat sink. All module electrical tests can be performed prior to heat sink mounting and encapsulation.
If the control circuit is elaborate and control components are desirable on the bottom surface of the heat sink, the heat sink can be undercut in these areas to provide the necessary space.
It will be noted that the substrate in the present invention is not glued to a shell and pressed into contact with the heat sink, but is adhesively attached directly to the heat sink for improved thermal characteristics.